The present invention relates to display devices and methods of fabrication, and more particularly to liquid crystal display devices and methods of fabricating liquid crystal display devices.
In order to minimize the space required by display devices, research into the development of various flat panel display devices such as LCD display devices, plasma display panels (PDP) and electro-luminescence displays (EL), has been undertaken to displace larger cathode-ray tube displays (CRT) as the most commonly used display devices. Particularly, in the case of LCD display devices, liquid crystal technology has been explored because the optical characteristics of liquid crystal material can be controlled in response to changes in electric fields applied thereto.
At present, the dominant methods for fabricating liquid crystal display devices (LCD) and panels are methods based on amorphous silicon (a-Si) thin film transistor (TFT) technologies. Using these technologies, high quality image displays of substantial size can be fabricated using low temperature processes. As will be understood by those skilled in the art, conventional LCD devices typically include a transparent (e.g., glass) substrate with an array of thin film transistors thereon, pixel electrodes, orthogonal gate and data lines, a color filter substrate and liquid crystal material between the transparent substrate and color filter substrate. The use of a-Si TFT technology typically also requires the use of separate peripheral integrated circuitry to drive the gates and sources (i.e., data inputs) of the TFTs in the array. Therefore, there is typically provided a large number of pads for connecting the gate lines (which are coupled to the gates of the TFTs) and data lines (which are coupled to the sources of the TFTs) to the peripheral drive circuitry.
FIG. 1 is a diagram illustrating a schematic layout of a conventional LCD display device. Here, plurality of gate lines 3 and plurality of data lines 7 are arranged in a substrate 1 in a matrix format. A plurality of gate pads 5 and a plurality of data pads 9 are also provided at ends of the gate lines 5 and the data lines 7, respectively. A portion of the device enclosed by one gate line 3 and one data line 7 typically forms a pixel 11. In addition, FIG. 2 is a flowchart illustrating five steps of a conventional method of forming a TFT-LCD display device, and FIGS. 3-5 are sectional views illustrating a TFT-LCD manufactured by the conventional method of FIG. 2.
A conventional method for manufacturing a TFT-LCD display device will now be described with reference to FIGS. 2-5. First, a first metal layer, having a stacked structure including chromium (Cr) and an aluminum (Al) alloy, is formed on a transparent glass substrate 100 to a predetermined thickness. Then, the first metal layer is etched by a first photolithography process to form a gate electrode 10 and a gate line 10xe2x80x2 on a TFT portion and gate pad portion of the substrate 100 (step 101). Then, a layer (e.g., nitride layer) is deposited on the-entire surface of the substrate having the gate electrode 10 and the gate line 10xe2x80x2 thereon to form a gate insulation layer 12. An amorphous silicon layer and an impurity-doped amorphous silicon layer are then sequentially deposited on the gate insulation layer 12 to form an amorphous semiconductor layer. Next, the amorphous semiconductor layer is patterned by a second photolithography process, resulting in a semiconductor layer pattern 14 on the TFT portion of the substrate 100 (step 102).
Then, a second metal layer such as Cr is deposited on the entire surface of the insulation layer 12 and on the amorphous semiconductor layer pattern 14 to a predetermined thickness. The second metal layer is then patterned by a third photolithography process to form a data line 16a and a source/drain electrode 16b on the TFT portion of the substrate, a gate pad 16c on the gate pad portion of the substrate, and a data pad 16d on a data pad portion of the substrate (step 103), as illustrated by FIGS. 3-5, respectively.
A passivation layer 18 is then formed on the entire surface of the above structure to a predetermined thickness. The passivation layer 18 is then patterned to expose parts of the drain electrode 16b, the gate line 10xe2x80x2 and data pad 16d using a fourth photolithography process (step 104). After forming an indium-tin-oxide (ITO) layer as a transparent conductive layer on the entire surface of the structure having the passivation layer pattern 18 thereon, the ITO layer is patterned by a fifth photolithography process to form a pixel electrode 20 (step 105).
Unfortunately, the use of chromium (Cr) as the second metal layer may not be preferred as a data line material because it typically has a relatively high resistivity. This relatively high resistivity can lead to an increased RC delay associated with the data line and can reduce the maximum viewing angle of the display. The use of chromium as the second metal layer may also be limited by the frequency of formation of metal line discontinuities during processing which can reduce device yield.
Also, the use of aluminum (Al) or an alloy thereof may not be preferred because contact formation between aluminum based alloys and indium-tin-oxide (ITO) layers typically results in the formation of aluminum oxide clusters. These oxide clusters typically act as electrical insulators and increase contact resistance. As will be understood by those skilled in the art, these insulating clusters are typically formed when current passes through the aluminum/ITO contacts and causes aluminum atoms to migrate into the ITO. This parasitic phenomenon is typically referred to as xe2x80x9cmetal migrationxe2x80x9d.
Thus, notwithstanding the above described method of forming TFT-LCD devices, there continues to be a need for improved methods of forming TFT-LCD display devices.
It is therefore an object of the present invention to provide improved thin-film transistor display devices and methods of forming same.
It is another object of the present invention to provide thin-film transistor display devices which are less susceptible to parasitic metal migration, and methods of forming same.
It is still another object of the present invention to provide thin-film transistor display devices having improved electrode and display characteristics, and methods of forming same.
These and other objects, features and advantages of the present invention are provided by thin-film transistor display devices having improved composite electrodes which provide, among other things, low resistance contacts and paths for electrical signals and are less susceptible to parasitic metal migration which can limit display quality and lifetime, and methods of forming same. In particular, a thin-film transistor (TFT) display device is provided having an insulated gate electrode on a face of a substrate (e.g., transparent substrate) and a semiconductor layer on the insulated gate electrode, opposite the face of the substrate. Spaced apart source and drain electrodes are also provided on the semiconductor layer. These source and drain electrodes each preferably comprise a composite of at least two layers containing respective metals therein of different element type. Preferably, one of the layers comprises a metal which is capable of forming a low resistance contact with electrodes such as a pixel electrode (e.g., transparent indium-tin-oxide electrode) and the other of the layers comprises a relatively low resistance metal so that the overall effective resistance of each composite electrode is maintained at a low level.
According to one preferred embodiment of the present invention, an insulated gate electrode is provided which contains a composite gate electrode on a face of a substrate and a gate insulating layer on the composite gate electrode. Here, the composite gate electrode preferably comprises a first gate layer containing a refractory metal such as chromium (Cr), molybdenum (Mo), titanium (Ti) and tantalum (Ta), and a second gate layer containing aluminum on the first gate layer. A semiconductor layer comprising a composite of two amorphous silicon (a-Si) layers is also provided on the gate insulating layer, opposite the face of the substrate. Here, the composite semiconductor layer preferably comprises an undoped first amorphous silicon layer having a thickness of about 2000 xc3x85 and a second doped (e.g., N-type) amorphous silicon layer having a thickness of about 500 xc3x85 on the first amorphous silicon layer. Spaced apart source and drain electrodes are also provided on the composite semiconductor layer, in ohmic contact with second amorphous silicon layer. These spaced apart source and drain electrodes define a channel region in the semiconductor layer which extends opposite the insulated gate electrode. Here, the source and drain electrodes each comprise a first metal layer containing a refractory metal and a second metal layer containing aluminum. The second metal layer preferably has a lower resistivity than the first metal layer to provide a low overall electrode resistance, however, the first metal layer preferably allows for the formation of low resistance ohmic contacts thereto which are less susceptible to metal migration and oxide cluster formation. Such contacts include an ohmic contact between the drain electrode and an indium-tin-oxide pixel electrode.
According to another embodiment of the present invention, a method of forming a thin-film transistor display device is provided which comprises the steps of forming an insulated gate electrode on a face of a substrate and then forming a semiconductor layer on the insulated gate electrode, opposite the face. Spaced apart composite source and drain electrodes are then formed on the semiconductor layer. These source and drain electrodes preferably each comprise a composite of at least two layers containing respective metals therein of different element type. In particular, one of the layers is provided so that each composite electrode has low overall resistance and the other of the layers is provided so that low resistance ohmic contacts can be formed thereto. According to this preferred embodiment, a pixel electrode (e.g., indium-tin-oxide) is also formed in ohmic contact with the one of the layers in the composite drain electrode. Here, for example, the drain electrode is formed as a composite of a patterned first metal layer comprising a refractory metal and a patterned second metal layer thereon containing aluminum. The pixel electrode is then preferably formed by removing a portion of the patterned second metal layer to expose the patterned first metal layer and then depositing indium-tin-oxide onto the exposed first metal layer. In addition, to provide low overall resistance, the second metal layer is formed having a thickness of about 2000 xc3x85 and the first metal layer is formed having a thickness of about 1000 xc3x85 so that the resistivity of the composite of the first and second metal layers is dominated by the resistivity of the second metal layer which is typically lower than the resistivity of the first metal layer.